GTP cyclohydrolase (GTP-CH) catalyzes the initial and regulatory step of tetrahydrobiopterin (BH-4) synthesis. BH-4 is the essential cofactor for the hydroxylation of phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp). It may regulate Phe hydroxylase activity by acting as a negative effector of its activation. In neurons, BH-4 may be responsible for the phosphorylation of Tyr and Trp hydroxylases, which enhances their activities. Induction of GTP-CH followed by elevation of BH-4 level and enhancement of Tyr hydroxylase activity have been reported. Just as Tyr hydroxylase is a marker for dopamine, GTP-CH is a marker for BH-4. BH-4 deficiency may reflect a defect of this enzyme and/or one of its biosynthetic enzymes. Variant forms of hyperphenylalaninemia have been found to be caused by a deficiency of BH-4. Reduced levels of BH-4 in cerebrospinal fluids from patients with Parkinson's disease, torsion dystonias, senile dementia of Alzheimer type, Steel-Richardson syndrome, and Huntington's chorea have also been reported. Monotherapy with BH-4 has proven successful in some patients with these disorders. Mammalian GTP-CH may be an aggregate of enzymes, but the enzyme complex itself has not yet been systematically studied. The research plan proposes the following studies: 1. Purify GTP-CH to homogeneity from liver and brain of rat and human. 2. Study the enzyme properties and protein structure of the multiple forms of GTP-CH. 3. Investigate the in vitro and in vivo regulation of GTP-CH activity by endogenous and exogenous compounds. This study should generate important basic information on the chemistry and function of mammalian GTP-CH, and the co-regulation of GTP-CH activity and BH-4 levels in rat brain. This information should also shed light on the role of GTP-CH and/or BH-4 in the development and treatment of monoamine-related neurological and psychiatric disorders.